Portosystemic Liver Shunts in Dogs: Why Small Breeds Fail to Thrive

If your small breed puppy seems smaller than their littermates, acts "spacey" after meals, or never quite grew the way you expected, a portosystemic shunt (PSS) may be the reason. This is a blood vessel abnormality where blood bypasses the liver instead of flowing through it, and it's one of the most commonly missed diagnoses in young dogs who fail to thrive.

The good news: once we diagnose a shunt, there are real treatment options. The key is recognizing when a "runty" puppy or a dog with weird neurologic episodes needs more than just time to grow.

What is a portosystemic shunt?

Normally, blood from the intestines flows through the portal vein into the liver before returning to the heart. The liver acts as a filter, removing toxins (especially ammonia from protein digestion) and processing nutrients. A portosystemic shunt is an abnormal blood vessel that lets blood skip the liver entirely, dumping unfiltered blood directly into systemic circulation.

The result: toxins that should be removed by the liver, particularly ammonia and other byproducts of protein digestion, build up in the bloodstream. These toxins affect the brain, causing neurologic signs. The liver also doesn't get the blood flow it needs to grow and function properly, so it stays small and underdeveloped.

Congenital vs. acquired shunts

Congenital shunts are present at birth. These are by far the most common type in small breed dogs, and they're what I'm usually looking for in a young dog who isn't thriving. The shunt vessel is a developmental abnormality, often a single vessel that should have closed before birth but didn't.

Acquired shunts develop later in life, usually as a consequence of severe liver disease that causes portal hypertension (high pressure in the portal vein). The body grows new vessels to relieve the pressure, but these vessels bypass the liver. Acquired shunts are multiple small vessels rather than one large one, and they're a sign of underlying liver failure rather than a primary problem.

Which breeds are most at risk?

Congenital portosystemic shunts show a strong breed predisposition. The breeds I see most often with this condition:

• Yorkshire Terriers (the highest risk breed)
• Maltese
• Miniature Schnauzers
• Shih Tzus
• Pugs
• Havanese
• Cairn Terriers

Large breed dogs can have shunts too, but it's less common. When large breeds do have congenital shunts, they're more often intrahepatic (the abnormal vessel is inside the liver), while small breeds typically have extrahepatic shunts (the vessel is outside the liver). This distinction matters for surgery.

What signs should make you suspect a liver shunt?

The classic presentation is a small breed puppy who's smaller than expected, acts strange after eating, and just doesn't seem "right." But the signs can be subtle, and owners often adapt to them without realizing something's wrong.

Growth and development

• Significantly smaller than littermates or breed standard ("the runt")
• Poor muscle development
• Thin, poor coat quality
• Failure to gain weight despite eating

Neurologic signs (hepatic encephalopathy)

This is often what brings dogs in. Signs tend to be worst 1-2 hours after eating, when ammonia levels peak:

• Staring into space, appearing "dull" or unresponsive
• Circling or head pressing against walls
• Seizures (sometimes the first obvious sign)
• Disorientation, especially after meals
• Behavior changes: unusual aggression or anxiety
• Blindness that comes and goes

Urinary signs

• Increased drinking and urination
• Ammonium biurate crystals or stones in the urine (a big red flag)
• Recurrent urinary tract infections

Gastrointestinal signs

• Vomiting
• Diarrhea
• Poor appetite
• Slow recovery from anesthesia or sedation (the liver can't process drugs normally)

If you're reading this because your Yorkie or Maltese puppy seems small and occasionally acts "spacey," that's worth investigating. These dogs often get dismissed as just being small or having a quirky personality.

How do we diagnose a portosystemic shunt?

Diagnosis involves bloodwork, sometimes special blood tests, and imaging to actually see the abnormal vessel.

Routine bloodwork clues

Standard bloodwork often shows hints that something's wrong with the liver:

• Low BUN (blood urea nitrogen): The liver converts ammonia to urea, so low BUN suggests poor liver function
• Low albumin: The liver makes albumin; a small, underfunctioning liver makes less
• Low cholesterol
• Mild anemia (microcytic, meaning small red blood cells)
• Liver enzymes may be normal or only mildly elevated

If I see a young small breed dog with low BUN, low albumin, and neurologic signs, a shunt is high on my list.

Bile acids test

This is the screening test for shunts. We measure bile acids in the blood before feeding (fasting) and 2 hours after a meal. Normally, bile acids are recycled through the liver. When blood bypasses the liver, bile acids leak into systemic circulation and levels become elevated, especially after eating.

A dog with a shunt typically has markedly elevated post-prandial (after meal) bile acids. This test doesn't confirm a shunt on its own, but it tells us liver function is compromised and we need imaging.

Ammonia levels

Fasting and post-prandial ammonia levels can also be measured. Elevated ammonia confirms that the brain is being exposed to toxins the liver should be removing. Ammonia testing requires careful sample handling (it must be run quickly), so it's not always available at every clinic.

Imaging: finding the shunt

Abdominal ultrasound is usually the first imaging step. A skilled ultrasonographer can often identify the abnormal vessel, see that the liver is small, and sometimes detect the characteristic changes in blood flow. However, shunts can be tricky to find on ultrasound, and not every ultrasound catches them.

CT angiography (CT with contrast) is the gold standard for confirming a shunt and mapping exactly where it is. This is essential information if surgery is being considered. CT provides a three-dimensional map of the abnormal vessel, showing the surgeon exactly what they're dealing with.

A portal scintigraphy (nuclear scan) can also confirm abnormal blood flow patterns but is less commonly available.

Treatment options: medical vs. surgical

Treatment depends on the type of shunt, the dog's overall condition, and what's realistic for the owner. The two approaches are medical management and surgery, and most dogs need some combination.

Medical management

Medical management doesn't fix the shunt, but it controls the symptoms by reducing the toxins that reach the brain:

• Low protein diet: Less protein means less ammonia production. Prescription hepatic diets (like Hill's l/d or Royal Canin Hepatic) are formulated for this.
• Lactulose: This medication acidifies the colon, trapping ammonia so it's excreted in stool rather than absorbed into the bloodstream. Most shunt dogs take this lifelong if not having surgery.
• Antibiotics (metronidazole or neomycin): Reduce ammonia-producing bacteria in the gut.

Medical management can work well for some dogs, especially those with smaller shunts or partial liver function. Some dogs live comfortable lives on medication alone. But medical management is treating the symptoms, not the cause, and many dogs eventually need surgery for the best outcome.

Surgical correction

Surgery aims to close the shunt, forcing blood to flow through the liver as it should. This is curative when successful, but it's specialized surgery that should be performed by a board-certified surgeon, ideally one with experience in liver shunts.

The challenge: you can't just tie off the shunt completely all at once. The liver has been underdeveloped because it hasn't received normal blood flow. If you suddenly redirect all that blood through the liver, the portal pressure spikes dangerously. Surgeons use devices like ameroid constrictors or cellophane bands that gradually close the shunt over weeks, giving the liver time to grow and adapt.

Surgical outcomes are generally good. According to the American College of Veterinary Surgeons, about 85% of dogs improve significantly after surgery, and many have complete resolution of their signs. Dogs that have surgery young (before severe liver damage or complications develop) tend to do best.

Surgery costs vary significantly depending on the facility and the complexity of the case. Expect $3,000 to $7,000 or more at a specialty center. CT imaging before surgery adds to that cost. This is a significant investment, but for a young dog who could have a normal lifespan with surgery, it's often worth pursuing.

What's the prognosis?

Prognosis depends on several factors:

• Single extrahepatic shunts (most common in small breeds) have the best surgical prognosis
• Intrahepatic shunts are more challenging surgically but still often correctable
• Multiple acquired shunts are not surgically correctable because the underlying liver disease is the problem
• Dogs diagnosed and treated early, before severe hepatic encephalopathy or urinary stones, do better

With successful surgery, many dogs go on to live normal lifespans. With medical management alone, quality of life can still be good for years, though most dogs require lifelong medication and dietary management, and some eventually develop complications.

When should you bring your dog in?

You should request an appointment if your small breed dog shows:

• Poor growth or being much smaller than expected
• Neurologic signs after eating (staring, circling, disorientation)
• Unexplained urinary crystals or recurrent UTIs
• Slow recovery from anesthesia or sedation

If your dog is having seizures, severe disorientation, or can't stand, that's an emergency and they need to be seen immediately (including emergency care if it's after hours).

For puppies who just seem smaller than expected without other concerning signs, a wellness visit with bloodwork is a reasonable starting point. If routine labs show the classic pattern (low BUN, low albumin, low cholesterol), we'll know to dig deeper.

If your small breed puppy isn't thriving

Portosystemic shunts are one of those diagnoses where catching it early makes a real difference. A puppy who's always seemed "off" or small for their breed isn't necessarily just a runt. If we can diagnose a shunt while the dog is young and otherwise healthy, treatment options are better and outcomes improve.

If you have a young Yorkie, Maltese, Schnauzer, or other small breed who isn't growing well or acts strange after meals, I'd encourage you to get them checked rather than waiting. Schedule a visit at Hearthstone Animal Clinic or give us a call at (281) 859-9244. We're here in Cypress, TX, and we can start with the right bloodwork to see if this is something we need to pursue further.